FIG. 1 is a block diagram illustrating a MIMO-OFDM system employing multiple transmission/reception (Tx/Rx) antennas.
A transmission end 1000 of the MIMO-OFDM system is shown in the left of the MIMO-OFDM system of FIG. 1. In this case, if the transmission end 1000 is an entity capable of transmitting a signal, it may be any one of a Node-B or a user equipment (UE). In the transmission end 100, a channel encoder 100 attaches redundant bits to a data packet to protect information bits from a negative influence of a channel or noise. A mapper 110 converts data bit information into data symbol information. A serial-to-parallel (S/P) converter 120 converts serial data into parallel data. A MIMO encoder 130 converts data symbols into space-time signals. The multi-antenna 140 for use in the transmission end 1000 transmit the space-time signals to channels.
A reception end 1500 of the MIMO-OFDM system is shown in the right side of the MIMO-OFDM system of FIG. 1. In this case, if the reception end 1500 is an entity capable of receiving a signal, it may be any one of a Node-B or a user equipment (UE). The Rx antenna receives a signal from a channel. A MIMO decoder 160 converts the received space-time signals into the data symbols. The de-mapper converts the data symbols into information bit. The P/S converter 180 converts a parallel signal into a serial signal. The channel decoder 190 decodes a channel code, and data is estimated at 195.
In the above-mentioned MIMO-OFDM system, space-time codes have been frequently used to maximize the spatial diversity gain. Several types of space-time codes that can provide considerable performance gain according to the number of transmit antennas and/or spatial multiplexing rate are shown in the following Table 1:
TABLE 1# of Txspace-time codes (Scheme)Rateantenna            (      1      )        ⁢                  ⁢                  1                  2                    ⁡              [                                                            S                1                                                                    -                                  S                  2                  *                                                                                                        S                2                                                                    S                1                *                                                    ]              ⁢                  ⁢    or    ⁢                  ⁢                  1                  2                    ⁡              [                                                            S                1                                                                    S                2                                                                                        -                                  S                  2                  *                                                                                    S                1                *                                                    ]              ⁢        12       (    2    )    ⁢          ⁢            1              2              ⁡          [                                                  S              1                                                                          S              2                                          ]      22                     (        3        )            ⁢                          ⁢                        1                                    2              ⁢                              (                                  1                  +                                      r                    2                                                  )                                                    ⁡                  [                                                                                          S                    1                                    +                                      jr                    ·                                          S                      4                                                                                                                                        r                    ·                                          S                      2                                                        +                                      S                    3                                                                                                                                            S                    2                                    -                                      r                    ·                                          S                      3                                                                                                                                        jr                    ·                                          S                      1                                                        +                                      S                    4                                                                                ]                      ⁢                  ,                  ⁢          r      =                          ⁢                        5                ±                  1          /          2                      ⁢        22             (      4      )        ⁢                  ⁢                  1        2            ⁡              [                                                            S                1                                                                    S                2                                                                    S                3                                                                    S                4                                                                                        S                2                *                                                                    -                                  S                  1                  *                                                                                    S                4                *                                                                    -                                  S                  3                  *                                                                                                        S                3                                                                    -                                  S                  4                                                                                    -                                  S                  1                                                                                    S                2                                                                                        S                4                *                                                                    S                3                *                                                                    -                                  S                  2                  *                                                                                    -                                  S                  1                  *                                                                    ]              ⁢        14             (      5      )        ⁢                  ⁢                  1                  2                    ⁡              [                                                            S                1                                                                    S                2                                                    0                                      0                                                                          -                                  S                  2                  *                                                                                    S                1                *                                                    0                                      0                                                          0                                      0                                                      S                3                                                                    S                4                                                                        0                                      0                                                      -                                  S                  4                  *                                                                                    S                3                *                                                    ]              ⁢                  ⁢    or    ⁢                  ⁢                  1                  2                    ⁡              [                                                            S                1                                                                    S                2                                                    0                                      0                                                          0                                      0                                                      S                3                                                                    S                4                                                                                        -                                  S                  2                  *                                                                                    S                1                *                                                    0                                      0                                                          0                                      0                                                      -                                  S                  4                  *                                                                                    S                3                *                                                    ]              ⁢        14             (      6      )        ⁢                  ⁢                  1        2            ⁡              [                                                            S                1                                                                    -                                  S                  2                  *                                                                                    S                5                                                                    -                                  S                  6                  *                                                                                                        S                2                                                                    S                1                *                                                                    S                6                                                                    S                5                *                                                                                        S                3                                                                    -                                  S                  4                  *                                                                                    S                7                                                                    -                                  S                  8                  *                                                                                                        S                4                                                                    S                3                *                                                                    S                8                                                                    S                7                *                                                    ]              ⁢        24
In the table 1, space-time codes (1), (2) and (3) are designed for 2Tx antenna system with different diversity and multiplexing tradeoff. In addition, space-time codes (4), (5) and (6) are designed for 4Tx antenna system. Rows and columns of the above-mentioned space-time codes may be differently used according to individual situations. Rows of individual space-time codes may be considered to be a space domain (i.e., transmit antenna) and columns may be considered to be a time (i.e., OFDM symbol) or a frequency domain (i.e., subcarrier) in OFDM based system.
Each space-time code of Table 1 may be represented by a Space-Time Block Code (STBC) or a Space-Frequency Block Code (SFBC). For example, the space-time code (5) of Table 1 may be represented by the following equation 1:
                    Transmit        ⁢                                  ⁢                  antenna          ↓                      1                          2                                      ⁢                              [                                                                                S                    1                                                                                        S                    2                                                                    0                                                  0                                                                                                  -                                          S                      2                      *                                                                                                            S                    1                    *                                                                    0                                                  0                                                                              0                                                  0                                                                      S                    3                                                                                        S                    4                                                                                                0                                                  0                                                                      -                                          S                      4                      *                                                                                                            S                    3                    *                                                                        ]                                →            Time                                              [                  Equation          ⁢                                          ⁢          1                ]                                Transmit        ⁢                                  ⁢                  antenna          ↓                      1                          2                                      ⁢                              [                                                                                S                    1                                                                                        S                    2                                                                    0                                                  0                                                                                                  -                                          S                      2                      *                                                                                                            S                    1                    *                                                                    0                                                  0                                                                              0                                                  0                                                                      S                    3                                                                                        S                    4                                                                                                0                                                  0                                                                      -                                          S                      4                      *                                                                                                            S                    3                    *                                                                        ]                                →                          Frequency              ⁢                                                          ⁢              or              ⁢                                                          ⁢              subcarrier                                                          [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 1, each column of the space-time code is considered to be a time, so that it is denoted by “STBC”. In Equation 2, each column of the space-time code is considered to be a frequency or a sub-carrier, so that it is denoted by “SFBC”.
The above Equation 1 indicates a scheme for transmitting symbols processed by individual columns in time domain. Therefore, in a high-speed mobility channel, inter-symbol interference could be significantly increased due to channel aging between two columns in Equation 1 and that may result in severe performance degradation.
The OFDM system of the above Equation 2 selects a space-time code for each sub-carrier using similar channel characteristics between neighboring sub-carriers, and transmits symbols processed by the selected space-time code to another Tx antenna. Compared with Equation 1, the space-frequency block code (SFBC) can acquire a higher performance under a high-speed mobile environment having a rapidly-changing channel.
The STBC expression of Equation 1 and the SFBC expression of Equation 2 can be applied to all kinds of space-time codes of Table 1.